gsteditor: avoid various compiler warnings and minor fixes
gsteditor: avoid various compiler warnings and minor fixes

 * added missing Gtk type casts
 * fixed use of gtk_message_dialog_new(): Caused problems if the dialog
   text contained magic characters like "%" and avoids a compiler warning.
 * fixed usage of datalists: This has probably fixed
   a segfault
 * comment out unused sort() callback
 * fixed one debug fprintf().
 * fixed setting back spinbutton if the canvas does NOT have a GstBin
 * Fixed adding elements in on_element_tree_select():
   Correctly check whether the Bin is paused now.
   Also avoids compiler warnings.

metratec/gst-editor,metratec/gst-editor,metratec/gst-editor,metratec/gst-editor